This invention relates to differential comparator amplifiers and more particularly to a differential amplifier which is suitable to be externally programmed to have symmetrical input hysteresis.
Differential amplifiers have been used for many purposes. One common use is as a zero cross detector for comparing differential input signals to produce a switching output signal in response to a predetermined relationship of the respective input signal. However, if there should be noise introduced onto the differential input signal, the zero cross detector or comparator may be falsely switched due to this noise and produce an erroneous output. For example, in contemporary anti-skid braking systems, a tachometer is utilized for receiving a generally sinusoidal input signal from a sensor pickup coil which is attached to a wheel of a vehicle. The frequency of this input signal is proportional to the rotational velocity of the wheel. The tachometer responds to the input signal to generate an output direct current (DC) signal, the magnitude of which is proportional to the frequency of the input signal. Thus, if a tachometer is used for each wheel concerned with, DC signals are generated which may be compared by suitable electronics to, for instance, indicate and cause the wheels to have substantially the same braking velocity to prevent skidding, as is known.
Basically, the tachometer comprises an input circuit, usually a zero cross detector or differential comparator amplifier which is responsive to the input sine wave from the sensor pickup coil for generating a rectangular wave output signal at the zero crossings of the input sine wave. Additional circuitry is utilized for generating output signals having a constant pulse width at the leading and trailing edges of the rectangular wave. These pulses are then integrated to provide the DC output signal. Hence, as the frequency of the input varies, the time between pulses varies which in turn linearly varies the DC output signal from the integrator. Thus, the magnitude of the output signal from the tachometer can be utilized to indicate the rotational velocity of the wheel involved.
A serious problem arises because of the noise introduced on the pickup of the sensor which modulates the sine wave signal that in turn is applied to the zero cross detector circuit. At or near the zero cross over transition point of the sine wave, this noise can cause false switching of the input circuit which produces an erroneous number of output pulses. Thus the integrator circuit will produce an erroneous DC output signal having a ripple introduced thereon which reduces the accuracy of the tachometer system.
The function of the zero cross detector or comparator circuit is to switch from one state to another as the input sine wave crosses a zero threshold voltage in either a negative or positive sense. If, the magnitude of the noise is great enough, as the zero threshold point is neared, the detector circuit could be caused to alternatively switch between states as the noise modulated sine wave input signal varies above and below the zero crossover axis. A solution to eliminate this problem may be provided by programming desired hysteresis to the detector circuit to increase and decrease the threshold point to overcome the noise transients. For instance, symmetrical hysteresis is provided, if the positive going threshold voltage is placed a given voltage level above the zero cross voltage and the negative going threshold voltage is placed the same given voltage level below the zero cross voltage. Thus, if the noise peak to peak level is less than the positive and negative threshold voltages, then this noise, when algebraically added to the sine wave input signal, is prevented from causing the output to be falsely switched.
Moreover, as the hysteresis is symmetrical, the output from the detector circuit remains symmetrical so that the output of the tachometer circuit remains linearly proportional to the frequency of the input sine wave.
Thus, a need arises for a circuit capable of externally and symmetrically programming hysteresis into a comparator circuit which is otherwise susceptible to noise transients introduced to the inputs thereof.